In an electrographic system image forming apparatus, after a photosensitive drum is charged negatively by a charge roller, an electrostatic latent image is formed when light is irradiated at the negatively charged portion of the photosensitive drum. The electrostatic latent image is developed by a developer supplied from a development roller and a supply roller, and a developer image produced by development is transferred onto a sheet by transfer rollers.
When the image forming apparatus is a printer forming color images, to faithfully reproduce a color image, there is a need to strictly control the amount of developer to be transferred onto a sheet. For example, Patent Document 1 discloses that the developer density of the patch pattern printed on a transfer belt is measured, and the processing conditions are controlled based on the density data obtained from the measurement.